1. Field of the Invention
The present invention relates to an optical head apparatus for recording/reproducing information by irradiating a light beam onto an optical recording medium such as an optical disk and an optical card and the like, and more particularly to an optical head apparatus which is capable of detecting a focus error without being influenced by noise, caused by diffraction from pits or recorded marks on a recording plane of the recording medium, or caused by diffraction from grooves on the recording plane of the recording medium while a light beam crosses across tracks thereof, thereby achieving a high utilization efficiency in detecting a focus error of the light beam.
2. Description of the Related Art
It is important, in an optical head apparatus for recording/reproducing information by using an optical disk, to coincide a focus position of an objective lens with a recording surface of the optical disk, so as to irradiate a light beam from a light source onto the recording surface. Therefore, detecting a focus error of the objective lens against the recording surface and thereby adjusting the objective lens along a beam axial direction of the objective lens is required as a control method called xe2x80x9cfocusing servoxe2x80x9d. There are many known ways of detecting the focus error, however, an astigmatic method is well known as one of the conventional ways thereof.
FIG. 1 is a diagram showing the system of detecting the focus error using the astigmatic method, as explained in Japanese Patent Disclosure No. H04-364231. The light beam generated from the light source 1 is condensed to the optical disk 6, through a collimator lens 2, a beam shaping prism 3, a beam splitter 4 and the objective lens 5. The reflected beam from the optical disk 6 is guided to a cylindrical lens 14 to give astigmatism, then the reflected beam is further guided to a four-divided photo-detector 15. The four-divided photo-detector 15 is arranged in a position where the section of the reflected beam becomes circular in an in-focus condition, that is, the condition when the focus position of the objective lens 5 coincides with the recording surface of the optical disk 6. The output from the four-divided photo-detector 15 is input to an arithmetic circuit 13 through a current-voltage convert amplifier array 12, and a calculation, such as (A+C)xe2x88x92(B+D), concerning each output of the four-divided photo-detector 15 is carried out in an arithmetic circuit 13, thereby obtaining a signal of the focus error.
In the focus error detecting system using the conventional astigmatism method, the focus error signal in the in-focus condition is always calculated to be zero, if the optical system is an ideal system and there is no misalignment between each optical element. However, the optical elements can not be disposed into the optical head apparatus without any misalignment. In case when there is misalignment of the optical element, the focus error signal in the in-focus condition is not calculated to be zero, under the influence of diffraction caused by pit or recorded mark, thereby making it difficult to realize a stable focusing servo.
FIG. 2A is a diagram showing the intensity distribution of the light beam on the four-divide photo-detector 15, and FIG. 2B is a diagram showing the movement of a light spot on the optical disk 6. Each figure is illustrated in case when the four-divided photo-detector 15 is disposed with misalignment. Here, each reference number 151 to 153 in FIG. 2A indicates the detecting surface of the four-divided photo-detector 15, and each circle thereof schematically shows the light beam entered at the four-divided photo-detector 15, and each dark portion thereof schematically shows the diffracted images generated by the pits or the recorded marks on the optical disk 6 respectively. Additionally, each circle 171 to 173 illustrated by short dashes of line shows the optical spot and the reference number 16 indicates the pit or the recorded mark on the optical disk 6.
As the optical beam, which is condensed by the objective lens 5, scans along the pits or recorded marks 16 on the optical disk 6 like spots 171, 172 and 173, as shown in FIG. 2B, the intensity distribution on the four-divided photo-detector 15 changes 151, 152 and 153 as shown in FIG. 2A. Here, the dark portions in the figure correspond to shade portions by the influence of diffraction caused by the pits or the recorded marks 16.
As shown in FIGS. 2A and 2B, when the four-divided photo-detector 15 has misalignment, the focus error signal F (=xe2x88x92(A+C)xe2x88x92(B+D) ) on the four-divided photo-detector 15 becomes positive in the intensity distribution on the receiving plane 151, or becomes zero in the intensity distribution on the receiving plane 152, or becomes negative in the intensity distribution on the receiving plane 153. Therefore, it is difficult to realize an accurate focusing control.
Further, there is another case in which the influence of the diffraction caused by grooves on the optical disk exists, other than those caused by the pits and recorded marks.
That is, in the focus error detecting system in the conventional astigmatism method, the focus error signal becomes zero all the time in the in-focus condition. However, the optical elements in the optical head apparatus can not be fixed without misalignment. In the case where such misalignment exits, the focus error signal does not become zero, by influence of the diffraction caused by the grooves, thereby making it difficult to realize the stable focusing servo.
FIG. 3A is a diagram showing the intensity distribution of the light beam on the four-divide photo-detector 15, and FIG. 3B is a diagram showing the movement of a light spot on the optical disk 6. Each figure is illustrated for the case when the four-divided photo-detector 15 is disposed with misalignment, similar to FIGS. 2A and 2B. Here, each reference number 251 to 253 in FIG. 3A indicates the detecting surface of the four-divided photo-detector 15, and each circle thereof schematically shows the light beam entered at the four-divided photo-detector 15, and each dark portion thereof schematically shows the diffracted images generated by the pits or the recorded marks on the optical disk 6 respectively. Additionally, each circle d to f shows the optical spot and the reference number 116 indicates the grooves on the optical disk 6.
The intensity distribution on the four-divided photo-detector 15 changes as reference numbers 251 to 253 as shown in FIG. 3A by scanning the beam spot, condensed by the objective lens 5, as d, e, and f on the grooves 116 in FIG. 3B. Here, the dark portions thereof show the diffracted images generated by the grooves 116.
As shown in FIGS. 3A and 3B, when the four-divided photo-detector 15 has misalignment, the focus error signal F (=(A+C)xe2x88x92(B+D) ) on the four-divided photo-detector 15 becomes positive in the intensity distribution on the receiving plane 151, or becomes zero in the intensity distribution on the receiving plane 152, or becomes negative in the intensity distribution on the receiving plane 153. Therefore, it is difficult to realize an accurate focusing control.
The present invention has been made in view of the above-mentioned circumstances and is intended to solve the above-mentioned problems. In particular, one purpose of the present invention is to provide an optical head apparatus in which the focus error signal thereof is less influenced by diffraction caused by the pits or the recorded marks, or caused by the grooves, on the optical disk, even though the optical elements have misalignment.
The present invention provides an optical head apparatus that is capable of reproducing information from an optical recording medium, including: an objective lens that receives a light beam from a light source and condenses the light beam onto the optical recording medium; a photo-detector that has plurality of photo-detecting planes disposed orthogonal to an optical axis of the reflected light beam and symmetrical with respect to the optical axis, and detects the light beam reflected by the optical recording medium, wherein each of the photo-detecting planes is divided into a plurality of divided areas by a plurality of dividing lines which symmetrically divide the photo-detecting planes with respect to the optical axis; a diffractive optical element that is disposed upstream to the photo-detector for diffractive the reflected light beam, wherein the diffracting optical element diffracts the reflected light beam such that the reflected light beam is divided at least in a direction parallel to a track of the optical recording medium, and a +Nth order diffracted light beam and xe2x88x92Nth order diffracted light beam (N is an integer greater than or equal to 1) in the photo-detecting plane are independently received by the divided areas which are disposed in positions symmetrically opposite with respect to the optical axis respectively; an arithmetic circuit that calculates an error signal with respect to a position of the objective lens along the optical axis, by using outputs from the photo-detector concerning both the +Nth order diffracted light beam and xe2x88x92Nth order diffracted light beam; and a driver that generates a signal for controlling the position of the objective lens based on an output from the arithmetic circuit.
The diffracting optical element may diffract the reflected light beam such that the reflected light beam is further divided in a direction orthogonal to the track of the optical recording medium.
The dividing lines of the photo-detecting plane may include at least one dividing line which extends in a direction parallel to the track of the optical recording medium. Here, the dividing lines of the photo-detecting plane may further include another dividing line which extends in a direction parallel to the track of the optical recording medium, and the dividing lines of the photo-detecting plane may further include another dividing line which extends in a direction orthogonal to the track of the optical recording medium.
The track on the optical recording medium may include a plurality of pits.
The optical head apparatus may further include: a beam splitter that guides the light beam to the objective lens when the light beam enters from the light source through a first optical path, and guides the light beam to the photo-detector when the light beam enters from the objective lens through a second optical path; and a beam shaping prism that is disposed in one of the first and the second optical paths, for modifying the sectional shape of the light beam.
The present invention further provides an optical head apparatus that is capable of reproducing information from an optical recording medium, including: an objective lens that receives a light beam from a light source and condenses the light beam onto the optical recording medium; a photo-detector that has a plurality of photo-detecting planes disposed orthogonal to an optical axis of the reflected light beam and symmetrical with respect to the optical axis, and detects the light beam reflected by the optical recording medium, wherein each of the photo-detecting plane is divided into a plurality of divided areas by a plurality of dividing lines which symmetrically divide the photo-detecting planes with respect to the optical axis; a diffractive optical element that is disposed upstream to the photo-detector for diffracting the reflected light beam, wherein the diffracting optical element diffracts the reflected light beam such that the reflected light beam is divided at least in a direction orthogonal to a track of the optical recording medium, and an +Nth order diffracted light beam and xe2x88x92Nth order diffracted light beam (N is an integer more than 1) in the photo-detecting plane are independently received by the divided areas which are disposed in positions symmetrically opposite with respect to the optical axis respectively; an arithmetic circuit that calculates an error signal with respect to a position of the objective lens along the optical axis, by using outputs from the photo-detector concerning both the +Nth order diffracted light beam and xe2x88x92Nth order diffracted light beam; and a driver that generates a signal for controlling the position of the objective lens based on an output from the arithmetic circuit.
The diffracting optical element may diffract the reflected light beam such that the reflected light beam is further divided in a direction parallel to the track of the optical recording medium.
The dividing lines of the photo-detecting plane may include at least one dividing line which extends in a direction orthogonal to the track of the optical recording medium. Here, the dividing lines of the photo-detecting plane may further include another dividing line which extends in a direction orthogonal to the track of the optical recording medium, and the dividing lines of the photo-detecting plane may further include another dividing line which extends in a direction parallel to the track of the optical recording medium.
The track on the optical recording medium may include a groove.
The optical head apparatus may further include: a beam splitter that guides the light beam to the objective lens when the light beam enters from the light source through a first optical path, and guides the light beam to the photo-detector when the light beam enters from the objective lens through a second optical path; and a beam shaping prism that is disposed one of the first and the second optical paths, for modifying the sectional shape of the light beam.
The present invention still further provides an optical head apparatus that is capable of reproducing information from an optical recording medium, including: a light source that emits a light beam; an objective lens that receives a light beam from a light source and condenses the light beam onto the optical recording medium; a photo-detector that detects the light beam reflected by the optical recording medium; a beam splitter that guides the light beam to the objective lens when the light beam enters from the light source through a first optical path, and guides the light beam to the photo-detector when the light beam enters from the objective lens through a second optical path; and a beam shaping prism that is disposed in one of the first and the second optical paths, for modifying the sectional shape of the light beam.
The optical head apparatus may further include a diffracting optical element that is disposed upstream to the photo-detector for diffracting the reflected light beam. Here, the diffracting optical element may be disposed between the objective lens and the beam shaping prism.